Most of the time, we want to make sure plastic stays out of water. It never biodegrades and is destroying our oceans and marine wildlife. But sometimes, it turns out, getting a little plastic in water can be a good thing. A new discovery has found a cheap and easy way to get arsenic out of water, and all you need is to pollute the contaminated water with a little plastic.

While it sounds like something out of an old movie, arsenic contamination of groundwater–whether from natural causes or man-made pollution–is painful and can be deadly. Getting arsenic out of the water traditionally takes expensive treatments, something that’s hard to come by in arsenic heavy areas like Bangladesh or Nepal.

Instead, this new technique involves taking chopped up bits of plastic and coating them with cysteine (an amino acid you can find in food or dietary supplements). The arsenic binds to the pieces of plastic. Pour them out and you have clean water. Lab tests found the method took 20 parts of arsenic per billion and reduced it to .2 parts per billion–making water that was twice the recommended limit for arsenic totally safe to drink.

The inclusion of cysteine means people in the developing world can’t implement this solution instantly in their homes, but it could be commercialized so people could buy cheap cysteine-coated plastic bits to drop in their water. At the very least, it would be a great way to get some use out of plastic otherwise headed toward a landfill.